FIG. 20 shows a conventional monopole antenna comprising one antenna element, which excites electromagnetic waves at two frequencies. In FIG. 20, numeral 91 denotes a disk-shaped earth conductor, numeral 92 denotes a current supply portion located at the center of the earth conductor 91, and numeral 93 is an antenna element made of a linear conductor. The antenna element 93 has a coil 93a at its center, and one end of the antenna element 93 is electrically connected to the current supply portion 92 located at the center of the earth conductor 91 so that it stands perpendicularly on the earth conductor 91.
In the antenna element 93, electromagnetic waves with lower frequency are excited in the entire antenna element, and due to the central coil 93a, electromagnetic waves with the same phase but higher frequency are excited above and below the coil 93a. Thus, a two-frequency monopole antenna oscillating at different frequencies can be obtained.
However, in this conventional monopole antenna, the height of the antenna element 93 has to be at least 1/4 of the wavelength at the lower excitation frequency or at least 3/4 of the wavelength at the higher excitation frequency, so that the antenna element 93 becomes relatively tall and miniaturization becomes difficult. Moreover, it is structurally impossible to excite electromagnetic waves whose frequencies are close to each other, so that the frequencies that can be excited are limited. In practice, operation of up to two frequencies is possible.
Furthermore, if this conventional monopole antenna is installed, for example, at the ceiling of a room, it is preferable that the antenna is installed head-down facing the floor, so that the antenna element 93 faces the space into which the electromagnetic waves are being radiated, in order to improve the radiation efficiency of the antenna. In this case, it is preferable that there are no objects hindering the transmission between the antenna element 93 and the radiation space, and that visual contact can be established between the antenna element 93 and the entire radiation space. Moreover, there is a need for monopole antennas that can be installed in a manner so that they can hardly be noticed, but if a conventional monopole antenna with an antenna element 93 protruding from the ceiling is used, its optical appearance is unpleasant, because the antenna element 93 is relatively tall.